It is very desirable to discriminate between normal cardiac rhythms and arrhythmias using a single, internally placed cardiac sensing lead. Previously, heart rate has typically been the criterion used for rhythm classification.
Using heart rate as the sole criterion for rhythm classification is problematic. Many physically active patients have heart rates during exercise that overlap with their tachycardia rates. Other patients exhibit supraventricular tachycardias, the rates of which overlap with rates of tachycardias of ventricular origin. These supraventricular tachycardias are often well tolerated and require no intervention.
When physicians classify an intracardiac rhythm, they examine the morphology of the electrocardiogram in addition to the heart rate. The shape of an intracardiac complex holds information on the origin and sequence of the heart's electrical activity. A normal intracardiac complex traverses the AV node and is conducted by specialized cardiac tissue throughout the ventricles This results in a distinctive complex morphology. A tachycardia of ventricular origin often has a very different morphology due to its ectopic origin and conductance through cardiac muscle tissue.
Sensing of the atria, in addition to the ventricles, would provide the most direct information on whether a complex was of normal or ventricular origin..sup.2 This requires another sensing lead and more complex sensing electronics. Currently available bradycardia pacemakers sense in both chambers but do so with the added cost of an additional sensing lead and device complexity. FNT 2. "Tachycardia Detection in Implantable Antitachycardia Devices", Jenkins J., Bump T., et al., PACE Vol. 7 November-December 1984 Part II.
The use of multiple leads in the ventricles has been proposed This would allow a determination of the ventricular activation pattern and provide more information on the source and sequence of ventricular electrical activity..sup.3 FNT 3. "Dual Lead Fibrillation Detection for Implantable Defibrillators via LMS Algorithm", DuFault RA, Wilcox AC, Proceedings of the Eighth Annual Conference of the IEEE/Engineering in Medicine and Biology Society.
The use of correlation programs to determine whether a complex matches a stored template has been proposed. This stored template is normally taken to be the normal intracardiac complex and a poor correlation to its shape is an indication of a tachyarrhythmia. These correlation programs require large amounts of computation and are unrealistic for the microprocessors and batteries used in today's implanted devices..sup.4 FNT 4. "Time Domain and Spectral Analysis of Electrograms in Man During Regular Ventricular Activity and Ventricular Fibrillation", Morkrid L, Ohm O, Engedal H, IEEE Transactions on Biomedical Engineering Vol. BME-31 No. 4, April 1984. FNT "Identification of Ventricular Tachycardia Using Intracavitary Ventricular Electrograms: Analysis of Time and Frequency Domain Patterns", Lin D, Dicarlo LA, Jenkins JM, PACE Vol. 11, November 1988.
Many template generating and comparison schemes have been proposed and patented..sup.5 Many of these are concerned with templates consisting of discrete samples of the signal's amplitude. FNT 5. Method and Apparatus for Comparing Waveform Shapes of Time-Varying Signals, U.S. Pat. No. 4,893,632. FNT Electrical Cardiac Defibrillator, U.S. Pat. No. 3,857,398. FNT System and Method for Predicting Ventricular Tachycardia by Derivative Analysis, U.S. Pat. No. 4,492,235. FNT Method and Apparatus for Analyzing Electrocardiographic Signals, U.S. Pat. No. 4,680,708. FNT Electrocardiogram Monitoring Apparatus, U.S. Pat. No. 3,829,766.
It has also been suggested that examination of the slopes of the peaks within the intracardiac complex can be used to discriminate between normal and tachyarrhythmic complexes..sup.6 FNT 6. "Detection of Pathological Tachycardia by Analysis of Electrogram Morphology", Davies DW., Wainwright RJ, et al., PACE Vol. 9 March-April 1986.
An existing defibrillator uses a probability density function (PDF) as an additional criterion for rhythm classification. This is a measure of the time the heart spends at electrical rest. PDF is not based on the morphology of complexes.
It is an object of the present invention to provide an improved medical device such as a cardiac pacer or a cardiac defibrillator, with morphology discrimination.
Another object of the present invention is to provide a device and method for discriminating between intracardiac electrical activity of supraventricular origin and intracardiac electrical activity of ventricular origin.
A further object of the present invention is to provide a self-contained implantable medical device for monitoring waveform complexes of intracardiac electrograms, in a highly effective and efficient manner.
Other objects and advantages of the present invention will become apparent as the description proceeds.